To me, this all comes down to balancing. It doesn't have to be "sharing means giving me what I want when I want it" and "first-come-first-serve means I can do whatever I want for as long as I want because I was here first."
I think that what the preschools are teaching in regards to sharing is not that kids don't have to share, but that sharing doesn't mean that the kids' right to use of an item is forfeited the second someone else says they want to use it. Just like having the right to first use, doesn't imply exclusive or excessive use.
Many posters have asked "what is excessive and what is reasonable"...and really that's a judgement call. The world isn't going to hand down this nice book of black and white rules...swinging is 30 minutes, soccer is 2 hours, etc. And I don't think etiquette is that stringent. If you're using the soccer field to play disc golf and someone else wants to use it for soccer, then just say "I'll be done at X time". I don't know the exact time that's reasonable, but I do know saying "I'm using it all day" or "I'm done when I'm done" isn't being considerate of other people's right to use of a public space.
And I do think that using equipment for other than it's intended purpose is fine until someone wants to use it for it's intended purpose. The example of the water aerobics in a swimming lane is a good one. To continue to monopolize a lane intended for fast lap swimming simply because they were there first is inconsiderate and rude.
Basically, no one has the right in polite society to blanketly trump someone else (excluding emergency situations)...if you're using something, then continue to use it and be cognizant of someone else's need/desire to use the same thing. Be fair, give yourself a reasonable time limit (and that's a judgement call on what that exact time is) and then turn over the item to the next person. That's what being polite and considerate is...it is not being a doormat.
And I really don't think we need laws and written rules/signs to tell us this. We should be able to use our good judgement independant of someone else telling us what to do....at least in this instance.